This invention relates generally to sensor devices that furnish continuous or analog outputs, indicating the pressure or temperature conditions experienced or sensed by a sensing element. In particular, the invention relates to removable and interchangeable sensor modules to be used as data input devices for modular switch system as described in U.S. Pat. No. 4,870,863 (hereinafter '863 patent).
Pressure switches are known that include an integral pressure sensing element connected to a pressure line. The output of the pressure switch indicates that the pressure in the pressure line stands above or below a selected set point or particular value by making or breaking contact between pairs of user-supplied wires. Temperature switches likewise are available to provide similar indications on user-supplied wires in response to sensed temperatures being above or below selected set points or particular values.
Such switches become used in such as industrial applications for controlling processes involving pressure and temperature. While these switch devices have in the past substantially fulfilled the requirements for indicating process parameters, they exhibit some drawbacks. In particular, the switch devices generally have been limited by the accuracy of the included sensing element. A more accurate pressure or temperature switch device required a more accurate and more expensive sensing element and a more accurate electrical circuit for converting the sensed pressure and temperature to the desired opening and closing of contacts for the output. If the range or type of switch device in its entirety was to be changed in the field, the old temperature or pressure switch device was removed and discarded and the new desired switch device was installed. This effected needless waste of an otherwise properly functioning switch device.
Additionally, pressure sensing elements usually provide a non-linear data signal to the control electronics of the switch device. This non-linearity previously became compensated or adjusted for by the control electronics before the device determined whether to change or maintain the condition of the output contacts. Normally, the sensing element data signal occurs at a low level and must be amplified for use by the control electronics. This introduces amplifier gain and drift and accuracy errors that can differ between different ones of the switch devices.
Pressure transducers, at least, have become known that include micro-electronic processors enclosed within the cylindrical case of the transducer. These transducers compensate for these non-linearities and different gain to provide a serial data signal indicating the pressure sensed by the pressure sensing element of the transducer. This overcomes some of the drawbacks of earlier switch devices, but again the entire case and contents becomes discarded when replaced by a switch device of a different range or of a different type, whether pressure or temperature.
The transducer calibration system disclosed in U.S. Pat. No. 4,446,715 to Bailey furnishes a transducer system capable of receiving data signals from a plurality of pressure transducers and that provides plural corresponding monitoring and correcting circuits to furnish a fully corrected pressure measurement output signal from each transducer.
The pressure transducers comprise a fiber optic pressure sensing element and a circuit card carrying opto-electronic signal producing components and an array of resistors. The resistors in the array identify the offset in the sensing element's output signal at zero pressure, the difference between the span of the sensing elements actual output signal and the span of its desired output signal and the degree of non-linearity in the sensing element's output signal.
The monitoring and correcting circuitry receives the data signal from the sensing element and can determine the resistance of the resistors in the array for correcting and compensating the sensing element to linear data. While the sensor elements or pressure transducers apparently can be connected to the monitoring and correcting circuitry as desired to obtain sensing of different pressure ranges, there is only one analog output, and the monitoring and correcting circuitry includes complicated compensation circuits. This system appears incapable, further, of accepting a temperature transducer in place of a pressure transducer.
The pneumatic-digital converter system and method disclosed in U.S. Pat. No. 4,483,178 to Mille furnishes a pressure to digital module assembly. A chassis of the system carries a plurality of pressure line connectors for the pressure lines to be monitored and carries a printed circuit mother board carrying a plurality of electrical connectors for receiving the digital data measured from the monitored pressure lines. Modules plug into the chassis and mother board for connection to the pressure lines and conductors carried on the mother board. Each module contains one or more pressure transducers, an analog to digital converter and a non-volatile memory component, which stores data to compensate for non-linearities, offsets, gains, etc. of the transducer and converter. Each module also contains a temperature transducer to sense the ambient temperature for correcting temperature variations in the pressure transducers. Changing the range of a pressure sensing element or changing to a temperature sensing module apparently requires discarding the entire module including the non-volatile memory, the analog to digital converter and control logic.
In the temperature type switch devices, it is known to provide a process monitor or meter that provides terminals for electrical connection to any one of several different types of thermocouples having different temperature ranges. These devices contain control logic and non-volatile memories to linearize the input signal depending upon the thermocouple type used. Switched and analog outputs are available apparently as factory options while some types of switch devices allow the user to change the type of output by simply changing an output circuit card. This later change becomes effected by removing the housing of the meter, sliding the output circuit board off the stakes connecting it to the control logic circuit board and installing the desired output circuit board. While the ability to change the type of outputs and the type of temperature sensors appears readily accomplished by such a device, such a switch device cannot readily be changed to sense and indicate pressures.
General purpose digital process monitors or controllers also are known to accept any one of two different types of sensing element data signals and to provide serial, discrete and analog outputs. Microprocessor based software performs wide ranges of mathematical operations on the data received from the sensing elements to perform desired linearization, scaling and range outputs. While such switch devices perform most switching functions conceivable, their expense prohibits them from being used in ordinary process control situations. Moreover, field changes appear to require opening the case of the switch device to interchange internal circuit boards.
What is desired is a switch device having a standard housing and standard, interchangeable sensor and output modules that serve well for processing pressure and temperature data from respective sensing elements and that can supply desired switched and analog outputs. Such a switch device could be manufactured with few differing parts to provide temperature and pressure sensing over wide ranges. This would simplify manufacturing to make specific ones of standard parts and to combine desired sensor and output modules with a standard housing assembly at final assembly to meet custom orders. In the field, customers could readily change between temperature or pressure ranges, between temperature and pressure sensing, and between desired outputs simply by removing and installing desired modules from exterior of the switch device housing.
Such a switch device could be manufactured inexpensively, and once installed in a customer's plant, could be changed as desired to meet changing process control needs. Such a switch, however, should be arranged and constructed for each sensing device to provide the most accurate data possible between manufacturing lots of the sensing elements of the same type and range.